Business tax cuts and proliferating shelters pushed the effective tax rate of 275 large U.S. corporations down by 20 percent since 2001, even as their pretax profits jumped 26 percent, according to a new analysis by Citizens for Tax Justice, a liberal tax watchdog group. Dozens of companies in the sample paid no taxes.

The study attempts to explain a sharp falloff in corporate tax receipts, which are at their lowest level in 20 years compared with the size of the economy, and which continued falling even as the economy rebounded in 2002 and 2003, according to the Congressional Budget Office.

But in targeting its analysis to the years President Bush has been in office, the group coupled its study with sharply partisan criticism of the administration's economic policies. The study focuses on the ways in which corporations can shelter their profits from taxes, an issue that predates Bush but which CTJ says has been increasing under the current administration.

Officials from several of the companies included in the study disputed the group's conclusions, and the Treasury Department blames recession and slow recovery for declining payments.

"The administration has aggressively pursued economic policies to get the economy back on track," said Treasury spokeswoman Tara Bradshaw.

The 275 companies reported almost $1.1 trillion in profit to their shareholders over the past three years, according to the group's analysis, which examined the Securities and Exchange Commission records of Fortune 500 companies that made money during that period.

The just over $200 billion that the companies paid to the Internal Revenue Service, the analysis said, yielded an effective tax rate of 18.4 percent on their total profit -- about half of the 35 percent corporate income tax rate. Over the three years, the effective rate dropped from 21.4 percent in 2001 to an average of 17.2 percent in 2002 and 2003.

The study did not contend that the companies broke any laws. But it noted that by taking advantage of new tax breaks for business investment, rising tax write-offs for employee stock options and a plethora of tax credits, the companies were able to shelter about half of their profits from taxation.

Citizens for Tax Justice is widely recognized as liberal-leaning, but its periodic corporate tax reports have proved influential in the past. One such analysis helped launch the tax reform movement of the 1980s.

Analysts for the organization said that the study released yesterday reflects broader trends in corporate taxation. That is unclear, however. The Congressional Research Service concluded in a recent study, for example, that the effective tax rate for all corporations was 24.6 percent -- substantially higher than the sample analyzed by CTJ.

In Wednesday's corporate income tax report, the first since 2000, the group and its affiliated Institute on Taxation and Economic Policy found that 82 of the 275 companies analyzed either paid no taxes or received large refunds in at least one of the past three years.

"The sharp increase in the number of tax-avoiding companies reflects the results of aggressive corporate lobbying and a White House and a Congress eager to do the lobbyists' bidding," said Robert S. McIntyre, director of Citizens for Tax Justice.

In 2003 alone, 46 companies with combined profit of $42.6 billion paid no federal income taxes and instead received rebates totaling $5.4 billion.

Twenty-eight companies had negative effective tax rates through the entire period, meaning that they received more in refunds and credits than they paid. Pepco Holdings Inc., the Washington utility, reported the lowest effective tax rate of the group, at minus-59.6 percent. Pepco paid no federal taxes on its $725 million in profit over the three-year period but received $432 million in tax "refunds."

Pepco Holdings received federal tax refunds in 2002 and 2003, in part because of large customer rebates the company then wrote off its taxes, said Jim Lavin, vice president and comptroller of Pepco Holdings. Pepco also benefited from tax breaks passed by Congress in the wake of the Sept. 11, 2001, terrorist attacks, he said.

But if the report looked at the company's large tax payments in 2000, the picture would look completely different, Lavin added.

"It's not like we're not paying taxes," he said.

Anne Eisele, a spokeswoman for the Boeing Co., said the report's numbers are flat wrong. Boeing paid $1.3 billion in federal taxes on $7.3 billion in earnings between 2001 and 2003, she said. According to the report, Boeing received cumulative tax refunds of more than $1 billion. Boeing received a $1.1 billion refund in 2003, but Eisele said that was an IRS repayment of tax overpayments from 1992 to 1997.

"The picture is a lot less clear than they are making it seem," she said.

The CTJ report said that actions by Congress and the Bush administration have caused corporate taxes to fall off.

In 1988, after a major tax-reform law closed huge corporate tax loopholes, the effective corporate tax rate stood at 26.5 percent. By 1998, that rate had slipped to 21.7 percent. And it has continued that slide, reaching 17.2 percent in 2002 and 2003, the CTJ said.

In 2002 and 2003, Congress passed business tax breaks worth $175 billion between 2002 and 2004, including a provision that allows companies to write off the cost of investments at a considerably faster pace. Twenty-five companies were able to claim $46.5 billion in tax savings from "accelerated depreciation" since 2001, led by SBC Communications Inc., which claimed $5.8 billion, according to the report.

Such tax breaks were supposed to spark corporate investment and jump-start the economy, but the study suggests the tax incentives "failed badly." The 275 companies in the study reported that their investment in property, plants and equipment had fallen by 15 percent since 2001.

SBC Communications claimed $5.8 billion in depreciation tax breaks between 2001 and 2003 but reported a 53 percent drop in new investments over that time.

John J. Stephens, vice president and controller at SBC, acknowledged that capital investment has fallen, but he blamed heavy regulatory disincentives that outweighed any tax breaks for the company's investment decisions.

"We carry an enormous tax burden for this country," he said, "and for that not to be made clear seems very inappropriate."